Genetic Linkage

I’ve just finished watching season 2 of Amazon video’s terrifying The Man in the High Castle, which depicts an alternate reality in which Germany and Japan won the second world war, with an atomic bomb taking out Washington, DC. I admit confusion over gaping plot holes and teasers, but I did catch enough of the dialog to suspect that genetic info about one of the families is wrong.

Thomas Smith, adolescent son of Obergruppenführer John Smith, has a form of muscular dystrophy. According to a footnote in Wikipedia, it is Landouzy-Dejerine syndrome, which I hadn’t heard of by that name but recognized its name today, facioscapulohumeral muscular dystrophy (FSHD). It is autosomal dominant – if you have it, so does or did one of your parents, unless you're a new mutation (about 30 percent of cases).

In a 10-second scene in the final episode, we learn that John Smith’s uncle had the disease, and that his two daughters must be carriers.

Well, no.

A dominant condition doesn’t have carriers. You have the mutation or you don't, although symptoms may occur later in life, in which case a person with the mutation is pre-manifest, not a carrier in the classic sense. Since Tom’s uncle had it too, presumably before the State murdered him, and the condition is rare, it must be inherited, not due to a new mutation. I can only reconcile these facts if John Smith has the mutation too but is non-penetrant – meaning he has the genotype but not the phenotype. I wonder what the Nazis, obsessed with purifying the gene pool, would do with that intel! Plus, Thomas has what appears to be seizures and he trips – not the upper body weakness of the disease.

I suppose the writers chose FSHD because it isn’t horrible, making it more horrible that the Nazis exterminate those who have it. When Thomas realizes that he’s not healthy, he mutters the mantras “I’m a useless eater” and “I'm defective.”

I won’t spoil the ending. But can anyone explain an inheritance pattern that has carriers and non-penetrance? Meanwhile, maybe the new showrunner for season 3 will take a genetics course.